There are a number of systems where the continued operation of a coil wound device, such as an electric motor or transformer is critical. For example, in an assembly line, an unanticipated failure of the electric motor results in the shut-down of the entire system. In these applications it would be desirable to have a warning of the imminent failure of the coil so that it could be replaced during a scheduled maintenance period for the device, rather than during operating time. One of the principal causes of coil failure is the failure of the insulation on the wire in windings. Over time, heat and vibration cause the electric insulation on the winding wire to break down, resulting in a short-circuiting of the winding.
Various attempts have been made to check or test the status of the insulation on windings. Most of these efforts focus on periodically applying a test voltage to the winding and measuring for leakage indicative of failed insulation. See, e.g., Horvath et al., U.S. Pat. No. 4,540,922, and Zelm, U.S. Pat. Nos. 5,155,441 and 5,172,289. However these methods usually require that the motor be taken out of service. Other efforts involved measuring the field properties of the winding, e.g. Palueff, U.S. Pat. No. 1,992,013 and Miller et al., U.S. Pat. No. 5,032,826, or measuring the motor operating variables, e.g., Kueck et al., U.S. Pat. No. 5,612,601. Still other efforts involved predicting failure by tracking the operating time and temperature of the motor, e.g., Chu et al, U.S. Pat. No. 5,019,760. Finally, sensing devices can be incorporated into the winding wire itself, e.g. Tokuda et al., U.S. Pat. No. 4,737,775. However, this could add significantly to the cost of the motor, and might affect operation of the motor.